There is set forth and described in my prior U.S. Pat. No. 4,694,943, a novel and improved means for converting a drive system of a vehicle between two-wheel and four-wheel drive and in such a way as to establish dynamic engagement or disengagement of a clutch assembly by remote activation. That system was designed to overcome problems associated with the prior art systems which are capable of maintaining four-wheel drive only when the engine is running.
It is recognized that a clutch engaging or disengaging under applied torque must provide high linear or axial forces to assure sufficient penetration of the teeth on a clutch or high interface forces on friction drive clutches. In the case of gear-type clutches, insufficient or only gear tip penetration under torque will tend to destroy the gears. Moreover, when a vehicle is loaded or has variable tire sizes or pressures, or is used off-highway, the engaged components of the power train are subjected to wind-up torque lock. Thus, the force necessary to separate the clutch gears is often greater than the original force required for engagement.
In addition, it has been proposed to employ vacuum systems as a means of engagement and disengagement of the clutch gears. However, such systems have not been entirely satisfactory from the standpoint of meeting the force and loading requirements in effecting engagement and disengagement. Among other problems, the applied vacuum within the wheel envelope must be maintained continuously during four-wheel drive operation and can impose external atmospheric pressures on the wheel seals beyond the capability of the seals.
It has also been proposed to employ an electrical heating unit to expand a chambered gas for driving a piston which then drives a fork against a clutch gear. However, among other things, systems of this type do not always function quickly and can be affected by wide swings in temperatures.
In U.S. Pat. Nos. 4,293,061 to Brown and 4,627,512 to Clohessy, power shift mechanisms are provided and which are mounted coaxially with respect to the clutch members but are powered in one direction only and must overcome a spring force acting in the opposite direction. In the '061 patent to Brown, it is necessary to compress the air in an envelope in order to shift in the one direction and the reverse spring pressure must then create a vacuum in order to return the envelope to its original state; and in both it is necessary to apply a continuous pressure or vacuum to maintain the clutch members in the engaged mode. U.S. Pat. Nos. 2,913,929 to Anderson and 4,271,722 to Campbell generally rely upon a power shift mechanism to effect engagement and disengagement of a clutch member but are not mounted coaxially with respect to the clutch member. Other representative patents in this field are U.S. Pat. Nos. 3,123,169 to Young et al. and 3,050,321 to Howe et al.